1. Field of the Invention
The present invention relates to fastening means for locking a turnable part, such as a screw, within a cylindrical cavity. It has particular application to the fastening of pump impellers and the like to drive shafts.
2. Description of the Prior Art
In order to attach a pump impeller to a shaft, the shaft and the impeller are provided with wedge-shaped grooves so that the impeller is unturnable relative to the shaft. In order to prevent the impeller from sliding off the shaft, a screw having a head larger in diameter than the shaft may be threaded into the shaft end. The screw is generally threaded so that when the shaft rotates, the screw strives to thread further in. If, however, a large object comes into the impeller which prevents it from rotating, it is possible that instead the screw may be threaded out, which may cause the impeller to come loose. This result may also be caused by vibrations in the pump.
In order to eliminate these risks, different types of screw locks have been applied such as jam nuts and the like. Jam nuts, however, have the disadvantage that they must be made relatively wide in order to permit necessary tightening force to be applied. Therefore, they encroach on the space in the impeller which may make an object in the pumped medium get caught.
Another way to provide a lock is by threading a screw into the shaft end. However, such a screw is difficult to loosen, which must be done in order to change the impeller and to perform other services on the pump.
The problems with locking are especially difficult in pumps of large size, where strong acceleration-retardation-and vibration forces appear.